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G. M. Keating (Geo. Wash. Univ./NASA Langley), A. Dwyer (Geo. Wash. Univ.), R. J. Wilson (GFDL/NOAA), R. H. Tolson (Geo. Wash. Univ.), S. W. Bougher, P. G. Withers (Univ. Ariz.), J. M. Forbes (Univ. Colo.)
In-situ measurements of over 1600 vertical structures of the Mars upper atmosphere have been obtained from the Mars Global Surveyor (MGS) Accelerometer Experiment during MGS aerobraking (1997-1999) compared to only 3 previous such vertical structures (Viking 1, Viking 2, and Pathfinder). Measurements of latitudinal and longitudinal variations in density, scale height, temperature and pressure have been obtained in the Northern and Southern Hemispheres on the dayside and in the Southern Hemisphere on the nightside. Enormous wave-2 longitudinal variations in density were discovered on the dayside with a nearly constant phase from 60 degrees N to 60 degrees S. The wave is observed throughout aerobraking near both winter and summer solstice. Nightside measurements obtained about 12 hours later in local solar time show a reversal in phase with wave-2 maxima occurring near where wave-2 minima occurs on the dayside. This phase reversal occurs at all altitudes measured on the day and night side, 120, 130, 140, and 150 km. Studies were made with the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) Mars General Circulation Model with a raised upper boundary to simulate the planetary-scale wave activity observed. It was found that most of the wave-2 features are apparently produced by a global-scale zonal wave 1 diurnal Kelvin wave. The GFDL simulation showed a wave-2 feature in fixed local solar time (1400) (similar to MGS) maximizing near 60 degrees E longitude and 240 degrees E longitude in accord with the phase of the observed wave on the dayside and reversing phase on the nightside in accord with observations. The wave was of a nearly constant phase from 60 degrees N to 60 degrees S in accord with observations. The phase of the wave remained nearly constant from near the surface to 130 km (near the limit of the model) increasing in amplitude with altitude. The wave moves in the opposite sense of the sun, to the east, rotating around the planet once per day. The non-migrating (or non-sun-synchronous) tide apparently results from modulation of the diurnal migrating tide by wave-2 topography.